Triple tags (called "machine tags" on Flickr and in some other places) are a way of tagging web content with tags having three parts: a namespace, a predicate and a value. For example:
taxonomy:binomial=Alcedo_atthis
They can be combined with hashtags:
#taxonomy:binomial=Alcedo_atthis
See Andy Mabbett's blog post about using Triple tags on Twitter.
Triple tags can also be used elsewhere, to refer to Twitter posts, or posters, see Triple-tag references to Twitter posts. Example:
twitter:status=1975532392